Load Freestyle script using api
The "script" property is a reference to a Text datablock.  You first need to create a Text datablock, and then set the "script" property to it.  Example:

import bpy
p = '/path/to/style_module.py'
t = bpy.data.texts.load(filepath=p)
m = bpy.context.scene.render.layers.active.freestyle_settings.modules.new()
m.script = t